1. Field of the Invention
The present invention relates generally to a mobile communication system based on an orthogonal frequency division multiplexing (OFDM), and more particularly to a method for estimating sequences of OFDM signals transmitted after quadrature amplitude modulation (QAM) and space-time block code (STBC) encoding operations are performed.
2. Description of the Related Art
It is difficult for 3G code division multiple access (CDMA) mobile communication systems to process large-capacity radio data. To overcome this difficulty, 4G orthogonal frequency division multiplexing (OFDM) mobile communication systems capable of processing large-capacity data on a multipath channel at a high speed have been proposed.
Conventionally, when data is transmitted over an orthogonal frequency division multiplexing (OFDM) system, sub-carriers carry symbols and then sub-channels combined with the sub-carriers are transmitted. Because spectrums of the sub-channels in the OFDM system can maintain the orthogonality with each other, and overlap each other, the efficiency of the spectrums is good. Because OFDM modulation and demodulation are implemented by an inverse Fourier fast transform (IFFT) and fast Fourier transform (FFT), a modulator/demodulator can be efficiently digitalized. Further, the OFDM system can efficiently operate in a current European digital broadcasting system, not affected by frequency-selective fading or narrow-band interference environments, and can also operate in a high-speed data transmission system based on standards of large-capacity wireless communication systems adopted by IEEE 802.11a, IEEE 802.16a, IEEE 802.16b, etc. Further, as the demand for broadband wireless communications increases in 4G mobile communication systems, research is currently being carried out on multiple-input and multiple-output (MIMO) systems using multiple antennas when transmitting and receiving operations are ongoing. In the MIMO systems, a decoding process uses a space-time block code (STBC), which is relatively uncomplicated.
FIG. 1 is a schematic diagram illustrating a conventional MIMO-OFDM transmission system. Referring to FIG. 1, data symbols to be transmitted by a transmitting side are modulated by a modulator (not shown). At this time, typical modulation schemes include quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM, etc. The above-described modulated symbols are input into an STBC encoder 110. The STBC encoder 110 carries out a space-time block encoding operation for the input modulated symbols. Then, outputs of the STBC encoder 110 are input into inverse Fourier fast transform (IFFT) processors 120, 121, . . . , 122. The IFFT processors 120, 121, . . . , 122 convert the outputs of the STBC encoder 110 into OFDM signals according to IFFT operations. Finally, the OFDM signals are transmitted through a plurality of transmitting antennas 130, 131, . . . , 132.
FIG. 2 is a schematic diagram illustrating a conventional MIMO-OFDM reception system. Referring to FIG. 2, a plurality of receiving antennas 201, 202, . . . , 203 receive OFDM signals. FFT processors 210, 211, . . . , 212 perform Fourier fast transform (FFT) operations for the received OFDM signals to generate FFT operation signals. A pilot detection and initial value estimation unit 220 produces an initial sequence estimation value using a pilot sub-carrier contained in each of the OFDM signals detected from the FFT operation signals and a predetermined initial value. Outputs of the pilot detection and initial value estimation unit 220 and FFT processors 210, 211, . . . , 212 are input into an STBC decoder 230. The STBC decoder 230 determines an optimum received signal and performs a space-time block decoding operation.
Methods for estimating the channel of a received signal in the OFDM system may include a deterministic maximum likelihood (ML) estimation method, a minimum mean square error (MMSE) estimation method, etc.
It is assumed that a channel impulse response is unknown but can be determined (not random) using the deterministic ML estimation method. The deterministic ML estimation method estimates the channel by obtaining an estimation value of the optimum ML. However, the deterministic ML estimation method has a problem in that the accuracy of estimation is degraded because statistical channel information is not used when the channel of a received signal is estimated. It is assumed that the channel impulse response is random in the MMSE estimation method. The MMSE estimation method estimates the channel such that the square of a channel estimation error value can be minimized. However, the MMSE estimation method has a problem in that it is difficult for the MMSE estimation method to be implemented in an actual system because statistical data associated with channel characteristics, signal-to-noise ratio (SNR) data, etc. are needed so that the channel of the received signal can be estimated.
As a result, the receiving side of the conventional MIMO-OFDM system needs an improved channel estimation method capable of compensating for the distortion of a signal due to multipath fading such that a QAM-modulated and space-time block coded signal can be appropriately detected.